Extruders commonly convert granular resin feeds into sheets, films and shapes such as pipes. Normally plastic pipe is produced by extruding molten plastic through a pipe shaping die orifice of an extruder. As used herein a pipe shaping die orifice is an elongated annular opening formed between an internal mandrel and a surrounding bushing which is spaced apart from the internal mandrel.
Since the viscosity of the molten plastic is temperature dependent, temperature measurement is an important operational guide to extruder operation. Temperature control schemes generally divide the extruder barrel into temperature control zones, with a temperature control loop including a controller, temperature sensor, heater and an over temperature alarm for each zone. Band-type electrical resistance heaters of two piece construction to facilitate removal are generally employed for directly heating the barrel.
Extruders require large heater ratings to decrease heating time. After extrusion has begun, however, heat is internally generated from friction, and at high production speeds the internal temperature, if uncontrolled, rises above the accepted maximum.
One of the factors which limits the production rate of an article formed by extruding a plastic material through an elongated die orifice is the rate at which heat can be removed from the article as it moves through the die. For an extrudate to be formed so that it is self-supporting when it emerges from the die, the plastic must be cooled below its solidification temperature before exiting the die. In the past this cooling has been accomplished by a variety of ways including, for example, circulating a cooling fluid through the outer portion or bushing of the die as well as through the mandrel. While cooling fluid circulation through the die, and other prior methods have proved effective for extruding relatively small diameter thin wall pipes, the prior art systems lack effectiveness in cooling larger diameter thicker wall pipes. This ineffectiveness in heat transfer from the plastic is due to the increased wall thickness of the large diameter pipe and thus lengthened heat conduction path for heat to escape from the extrudate so that heat is trapped around the mandrel of the die. The excessive temperature results in a condition known as control system temperature override which at least requires a reduction in extrusion speed and hence production rate to satisfy the temperature requirements.
Accordingly, it is an object of this invention to provide an improved method and apparatus for extruding plastic material.
Another object of this invention is to improve the rate of cooling the extrudate as it passes through the die of an extruder.
Still another object of this invention is to improve the production rate for extruding relatively large diameter pipes.
A yet more specific object of this invention is to provide a device for removing heat from the mandrel and from the inner surface of the extrudate passing from the die of an extruder.